The critical care community has been using severity and organ failure assessment tools for over 2 decades. The major adult severity assessment models are Acute Physiology and Chronic Health Evaluation, Simplified Acute Physiology Score, and Mortality Probability Model. All three recent versions of these models perform well in predicting hospital mortality. Sequential Organ Failure Assessment score is the most used tool for assessment of multiple organ failure. These tools have been used extensively in clinical research involving critically ill patients and for benchmarking and the measurement of performance improvement. Their roles as clinical decision support tools at the bedside await future studies because of their unknown or poor performance at the individual patient level.